As the Internet grows continuously, the data volume transported through the Internet become increasingly huge. Since conventional narrow band networks can not meet people's demands any more, many typical broadband technologies have emerged. Generic Framing Procedure (GFP), defined in ITU-T G.7041/Y.1303, is a new data link framing protocol mainly for block codes based on byte synchronous transmission channels or packet-oriented data streams.
Unlike High Level Data Link Control (HDLC) protocol that determines frame border by a frame header padded with specific characters, GFP adapts the frame positioning method in Asynchronous Transfer Mode (ATM) and employs a Header Error Check (HEC)-based self-description technology to determine frame border by two-byte of the current frame for payload length and two-byte of the current frame for HEC. Therefore, it overcomes defects in the frame positioning which depends on a frame tag and is suitable for high speed processing. Since the frame header in GFP is also a payload length indicator, the GFP frame encapsulation may be in a flexible form and support data in fixed or variable length, and can completely encapsulate the user's Protocol Data Units (PDUs) in variable length. Therefore, it avoids data parsing, restructuring and frame padding, simplifies operations, and improves system processing speed and stability.
Another feature of GFP is the introduction of IDLE frame: in the case that GFP doesn't receive any data from the service side, it will create an IDLE frame to solve the problem of mismatch between the service data rate and the data rate of lower transport service layer, and thereby avoid any limitation to the byte synchronous transmission on lower layer or the data service on upper layer. The IDLE frame feature of GFP makes it applicable to adaptation from any service to any byte synchronous stream when it is used as an adaptation layer (of course, the transmission bandwidth for byte synchronous stream on lower layer must be wider than that required by the service on upper layer), and reduces complexity resulted from FLAG byte application in Point to Point Protocol (PPP) and Link Access Procedure-SDH (LAPS). That feature is the basic reason why GFP is now widely applied.
In another aspect, to solve the problem regarding wavelength utilization, in a Wavelength Division Multiplexing (WDM) system, the conventional Optical Transponder Unit (OTU) used for point-to-point optical connections has been replaced by an optical transponder unit with multiplexing or add/drop multiplexing function. Although multiplexing multiple services to a single wavelength for transmission can improve wavelength utilization, such an application is still in a point to point application model, in which the wavelength utilization can be improved by service multiplexing only when there are multiple services between two points. However, such an application mode is rarely seen in practice.
In the applicant's Chinese patent application no. 200310121902.2 and entitled “Method for Improving Wavelength Utilization”, a method for improving wavelength utilization in WDM system by means of add/drop multiplexing is put forward. At present, there are two ways to implement add/drop multiplexing: Virtual Container (VC) and GFP Channel ID (CID). The VC method is to utilize the above-mentioned GFP or another similar adaptation protocol (e.g., PPP or LAPS) to encapsulate an upper layer service into virtual containers and then perform add/drop multiplexing on the VCs to implement add/drop multiplexing for the service. For example, a Gigabyte Ethernet (GE) signal can be encapsulated in GFP into a Virtual Concatenation Group (VCG) including 8 VC4s; then, those VCs are multiplexed into Synchronous Transfer Mode-N (STM-N) frames; at the destination node, the VCs are treated by add/drop multiplexing, so that the Add Drop Multiplexer (ADM) for the GE signal is implemented. For example, STM-16 data can be encapsulated into an Optical Data Unit of level 1 (ODU1); then, 4 ODU1s can be encapsulated into an ODU2; by treating the ODU1s through add/drop multiplexing, an STM-16 ADM function can be implemented. Compared with the VC method, GFP CID method is simpler. For example, before GEs are encapsulated into ODU1 frames, they are encapsulated into GFP frames first; by assigning different CIDs to different GEs and treating the GEs with different CIDs in different ways at the destination node (for example, a GFP frame with a CID identical to the GE to be dropped at the destination node is terminated at the node and dropped, and a GFP frame with a CID different from the GE to be dropped at the destination node will pass through that node and be transferred to the next node in the original direction), so as to implement the add/drop multiplexing for GEs.
Though both the above VC method and GFP CID method can implement ADM operations for services, they have their drawbacks respectively. In the VC method, separate VCs are assigned to each service. No matter whether a service makes full use of the corresponding VC bandwidth, the VC bandwidth is reserved for the service. According to a carrier's network including multiple GE ADMs, the typical bandwidth utilization for each GE is about 30%. Therefore, the bandwidth reserved for each GE is actually wasted and can not be used for other services. Though the VC bandwidth occupied by each service can be adjusted (e.g., a GE required 8 VC4s originally but now is adjusted to have 4 VC4s), that method has poor flexibility in that it can not provide enough bandwidth to GE services on upper layer at peak traffic times and also reduces the traffic through GE ports of routers on upper layer in disguised form, and the service capability of upper layer equipments are not fully utilized. Furthermore, the method of reducing VC bandwidth does not attain the purpose of utilizing “peak-valley effect” among the services to balance traffic demands among the services and implement statistical multiplexing. Though the GE ADM implemented with the GFP CID method enables multiple GE services to share the physical bandwidth of the same bearer pipe, the drawback of the GFP CID method is apparent, because the traffic for each GE is unpredictable and no traffic control mechanism is defined in GFP, and the existing implementation method actually reserves bandwidth for the sum of the respective peak traffics of GEs in the lower layer bearer pipe. Though the method simplifies the implementation of GE ADM, it reduces bandwidth bearing efficiency at individual wavelengths.